1. Field of the Invention
This invention relates generally to high voltage power distribution circuits in high density server power systems, and more particularly, to a start-up circuit and method for a high voltage power distribution circuit which improve the start-up of a metal-oxide-semiconductor field-effect transistor (MOSFET) into a capacitive load.
2. Description of Background
In high-density server power systems, for example, high voltage DC power distribution is controlled with electronic switches (e.g., MOSFETs) to isolate load faults and to provide desired system availability. Since loads of the MOSFET circuits are capacitive, high power dissipation occurs when the MOSFET is first enabled. Thus, using conventional methods, there is a continuous problem with the reliability of the MOSFET.
Generally, server power systems employ a conventional method for reducing power dissipation by stretching the start-up time (i.e., a soft start-up operation) of the high voltage power distribution circuit. Using this conventional method, stretching the start-up time results in decreasing the peak current which in turn decreases current accuracy and makes protecting the MOSFET more difficult. In addition, load circuits may not function properly, system timings are required to be altered, and power-on durations increase as a result of the longer start-up time. Further, when using this conventional start-up method, an initial peak MOSFET power spike is still present even though it is reduced in magnitude.
In the conventional start-up method for a high voltage power distribution circuit as mentioned above, a current source generates constant current which discharges a feedback capacitor of the circuit. The constant current is approximated by connecting a resistor to a 350V (for example) voltage source. The generated constant current first charges up a capacitor, which is connected with a non-inverting input of an operational amplifier connected between a MOSFET and the current source, until the non-inverting input of the operational amplifier is approximately at a gate threshold for the MOSFET. Upon starting up the MOSFET, the constant current has completely charged the capacitor and the current is then fed into the feedback capacitor discharging a voltage charge on the feedback capacitor. As shown in FIG. 1, the constant current forces the load voltage to increase linearly from zero to approximately 350V and the power generated inside the MOSFET is over approximately 300 W at start-up, thereby damaging the MOSFET when powering on the circuit. As illustrated, when employing this conventional start-up method, increasing the start-up time, for example, to above approximately 60 ms lowers the discharge current which in turn lowers the peak power of the MOSFET.
Accordingly, it would be desirable to have start-up circuit and method for controlling start-up of a high voltage power distribution circuit capable of reducing power dissipation upon start-up, while maintaining MOSFET reliability.